In a nuclear reactor, the fissionable material gradually becomes depleted to where it can no longer fuel a fission reaction, at which point the spent fuel must be removed and replaced. The spent fuel, which generally comprises a plurality of individual rods assembled in a bundle of substantially square cross-section, still may be highly radioactive, in which case it can be reprocessed to where it can again sustain or fuel a fission reaction.
It is generally necessary to ship the fuel over relatively long distances to the reprocessing plants, and thus it is essential that the spent fuel be packaged for shipping in such a manner that a high degree of safety, both to the outside world and to the rod assembly itself, is maintained. As a consequence, the rod assemblies are generally loaded into a basket which, in turn, is contained in a shipping cask. Such an assembly must be capable of preventing the escape of harmful radiation to the outside, but even more important, it must be capable of preventing neutron multiplication to a critical point through interaction among the several rod assemblies.
In addition to the neutron critically, the basket assembly must have sufficient structural strength to withstand sudden dynamic shocks such as occur in the fall of the cask onto an unyielding surface. Such a fall, for example, from a height of thirty feet, can create dynamic stresses that are catastrophic unless the basket has sufficient structural strength to withstand such impact loading.
Finally, the basket must be capable of transmitting fuel decay heat from the fuel assemblies to the cask walls with a degree of efficiency which will prevent heat buildup within the cask that exceeds safety limits. The Federal government, through the NRC, has specified a maximum interior heat of 380 degrees Celsius for storage purposes. For transport purposes, the allowable interior heat is approximately 535 degrees Celsius (1000 degrees Fahrenheit). Thus, any basket arrangement must be capable of conducting heat to the cask walls sufficient to maintain these or lower temperatures.
In U.S. Pat. No. 3,731,101 of Peterson et al, there is shown a shipping basket and cask arrangement that typifies prior art solutions to neutron criticality, strength and heat dissipation. Radiation shielding material completely fills the space surrounding the fuel assemblies, making a substantially solid assembly and heat radiating fins are mounted to the outer shell. Such an arrangement is both heavy and expensive to build, being prodigal in the use of radiation shielding material.
U.S. Pat. No. 4,177,938 of Heckman et al discloses a spent nuclear fuel cask arrangement which incorporates heat conducting fins into the structure thereof, and which does not require as much radiation shielding material as prior art devices typified by Peterson et al. However, the structure is not adapted to carry a plurality of fuel assemblies, and would require considerably more shielding if so adapted.
Some prior art devices utilize a grid lattice structure to carry the fuel elements, as shown in U.S. Pat. Nos. 4,066,500 of Woltron et al and 4,711,758 of Machado et al. Such grid structures tend to be lighter than other prior art arrangements, but their structural strength is not as great as the more massive prior art structures.
In U.S. Pat. No. Des 263,087 of Best et al, there is shown a basket arrangement in which the fuel assembly support means is also the heat radiating means, and the fuel containing members form a portion of the structure.
In all of the prior art arrangements, there is an interdependence among the three main desiderata of preventing neutron criticality in a structure capable of withstanding stresses and which has heat dissipation means. The more the accomplishment of these ends can be made independent of each other, the safer the overall structure becomes, since failure or inadequacy in accomplishing one end will not affect the accomplishment of the others.